It is well known that certain materials such as semiconductors are capable of acting as optical amplifiers. For example, when certain semiconductors, which exhibit a band gap, are subject to an injected electric current, an incident photon causes an electron to traverse the gap with the result that an additional photon is generated, thereby producing optical amplification. Semiconductor optical amplifiers and lasers which operate in this way are well known and reference is directed to "Long Wavelength Semiconductor Lasers" G. P. Agrawal and N. K. Dutta, Van Nostrand, Chapters 1 to 6.
The semiconductor material which is used as the active amplification region of the device suffers from a gain-saturation effect which imposes a limitation on the maximum power that can be obtained. In order to overcome this problem, proposals have been made in the past to provide the active amplification region as a tapered structure on the substrate, which widens along its length so that as amplification proceeds, a greater cross section of material is available for amplification, permitting increased amplification power to be achieved. Reference is directed to Mehuys et al: "525 W, CW Near Diffraction Limited Tapered Stripe Semiconductor Optical Amplifier", IEEE Phot Tech. Letts. 5 pp 1179-1182, 1993. In this arrangement, an expanding path through the active amplification region is achieved by means of an input lens arrangement and the resulting amplified light is collected by an output lens. Another arrangement with an expanding, tapered active region, but with an integrated laser source, is described in Parke et al "2.0 W CW Diffraction Limited Operation of a Monolithically Integrated Master Oscillator Power Amplifier", IEEE Phot Tech. Letts, 5 297-300 1993. Reference is also directed to a similar structure described in Bendelli et al "A New Structure for High Power TWSLA", IEEE Phot Tech Lets. 1, 1991, pp 42-44.
Reference is also directed to "Q-Switched Bow-Tie Lasers for High Energy Picosecond Pulse Generation," K. A. Williams et al, Elect Lett, February 1994, Vol 30 No. 4, pp 320-21 which illustrates outwardly tapering amplifier regions in a laser to avoid gain saturation. Reference is made to EP-A- 0 135 594 which discloses a semiconductor laser wherein tapering of an electrode is used to improve astigmatism.
In Koyama et al "Multiple Quantum Well GaInAs/GaInAsP Tapered Broad Area Amplifiers with Monolithically integrated Waveguide Lens for High Power Applications", IEEE Phot Tech Letts, 5 pp 916-919, 1993, there is described the use of an integrated lens to re-focus the output of the expanding, tapered active region in order to simplify coupling into a single mode fibre. However, in practice, difficulties arise in fabricating the integrated lens.